Field of the Invention
The present invention relates to a membrane for separating gases, said membrane being composed of a cellulose fiber that possesses different permeabilities to different gases, and to a process for preparing that membrane.
More particularly, the present invention relates to a hollow fiber having a high gas permeation speed, a selective gas permeability and a high pressure resistance, a hollow fiber as aforesaid coated with a polymeric substance, and to processes for the preparation of such hollow fibers.
As gas-separating polymeric membranes, there are known a so-called homogeneous film, a porous film consisting of an entirely porous structure and a so-called asymmetric film comprised of a very thin homogeneous layer (0.3 to 0.7.mu.) present as a surface layer on a porous supporting layer. Membranes composed of a homogeneous film or an asymmetric film have the inherent selective permeability of the polymer of which the film is made. In membranes of this type, it is known that the gas permeation speed or gas permeability is generally proportional to the film area and the pressure and is inversely proportional to the film thickness (the thickness of the very thin homogeneous layer in case of the asymmetric film).
In the case of a porous film, the gas permeation speed is proportional to the reciprocal of the square root of the molecular weight of the gas, and it is much higher than that of the homogeneous film. According to our experiments, it has been found that the gas permeation speed of a porous film is 10.sup.4 to 10.sup.7 times the gas permeation speed of a homogeneous film.
In view of the foregoing, it is recognized that in order to perform gas separation effectively, it is necessary to increase the permeation area and the pressure difference and to reduce the film thickness, when a homogeneous film is employed.
When a planar permeation membrane is employed, in order to enable the membrane to withstand a high pressure it is necessary to increase the membrane thickness and to provide a porous supporting layer.
These requirements can be satisfied by using a hollow fiber including an asymmetric film. More specifically, a hollow fiber having a small diameter has a higher pressure resistance and provides a larger surface area in comparison with the same weight of a planar film. Further, if the thickness of the homogeneous portion of the asymmetric film is reduced, the permeation speed can be increased.
However, it is very difficult to prepare an asymmetric hollow fiber, and in connection with all polymeric substances, asymmetric hollow fibers having a good selective gas permeability have hardly been known in the art.
According to a first embodiment of Ser. No. 725,509 there is provided a membrane composed of a hollow fiber of a cellulose derivative, which fiber has a very high gas permeation speed and a separating capacity substantially in proportion to the reciprocal of the square root of the molecular weight of the gas, and a process for preparing such a fiber.
However, it has been found that it is difficult to separate a mixture of gases in which the difference of the molecular weights is small, into respective gases at a high efficiency by using this membrane.
According to a second embodiment of Ser. No. 725,509 is provided a hollow fiber as in the above-mentioned first embodiment which is coated on its surface with a very thin homogeneous polymer film having a high gas permeability, and a process for preparing same. The fiber of the second embodiment is unexpectedly superior to the fiber of the first embodiment because it can effect separation of a mixture of gases having similar molecular weights with high efficiency.
Various semi-permeable membranes having a two-layer structure are known in the art. For example, Japanese Patent Application Laid-Open Specification No. 62380/74 discloses a hollow fiber semi-permeable membrane having a good reverse osmosis capacity, which is made by the concentric sheath-core composite spinning method and which comprises a core of a polymer composition providing a porous structure and a sheath of a polymer composition providing a compact structure. According to this method, however, a complicated apparatus must be used, and it is expected that a high operation control will be required. Further, Japanese Patent Publication No. 17589/73 discloses a semi-permeable membrane having a two-layer structure which is formed by coating a polymer solution on a polyolefin film having dispersed therein a salt which is soluble in water or the like and then removing the soluble salt by extraction. This method is different from our method in the point that in this conventional method, if the sequence of the step of coating the polymer solution and the step of removing the soluble salt by extraction is reversed, clogging occurs in the porous structue and a fiber having the intended properties cannot be obtained. There is also known a method in which a thin film is crosslinked and coated on a porous film by plasma polymerization. In view of difficulties involved in the apparatus and operation of this process, it is considered that this method is lacking in practical utility.
We have discovered, according to the first embodiment of Ser. No. 725,509, a process in which hollow fibers having a selective gas permeability and possessing various additional properties can be obtained by a simple procedure.
According to the second embodiment of Ser. No. 725,509, we have discovered that an unexpectedly improved hollow fiber having a selective gas permeability can be prepared by coating a polymer solution having a prescribed concentration onto the hollow fiber of the first embodiment, by dipping or the like, as illustrated in the Examples given hereinafter.
More specifically, the first embodiment of Ser. No. 725,509 provides a process for preparing hollow fibers having a selective gas permeability which comprises the steps of dissolving a cellulose derivative in a solvent to form a solution having a solid content of 22 to 33% by weight, extruding said solution from the annular space between the concentric inner and outer tubes of a double tube-type spinneret and simultaneously feeding an aqueous liquid or a gas from the interior of the inner tube of said spinneret directly into an aqueous coagulating bath maintained at 0.degree. to 45.degree. C. to form a hollow fiber, dipping the hollow fiber in warm water maintained at 30.degree. to 100.degree. C. without drying, taking the hollow fiber out of the water bath, removing water from the hollow fiber inclusive of the water present in interior hollow portion, and immediately freeze-drying the hollow fiber under vacuum to obtain the fiber of the first embodiment of Ser. No. 725,509.
The fiber of the second embodiment of Ser. No. 725,509 is prepared by coating the exterior surface of the thus-obtained hollow fiber with a solution formed by dissolving a polymer in a solvent which is incapable of dissolving said cellulose derivative so that the solid (polymer) content of this coating solution is 0.2 to 5.0% by weight, thereby to form on the surface of the hollow fiber a homogeneous and thin coating film of the polymer, said coating film having a thickness of 0.1 to 300.mu.. The hollow fiber of the first embodiment of Ser. No. 725,509 prepared according to the above-described process, namely, a hollow fiber having a selective gas permeability consists essentially of an asymmetric hollow fiber of a cellulose derivative, said asymmetric hollow fiber (a) having an outer diameter of from 0.2 to 3 mm and an outer diameter/inner diameter ratio of from 1.1 to 3.0/1 and (b) having a nitrogen gas permeability of 5 .times. 10.sup.-4 to 0.1 (cm.sup.3 (STP)/cm/.sup.2.sec.cmHg), wherein its permeability to various gases is substantially in proportion to the reciprocal of the square roots of the molecular weights of the respective gases.
The hollow fiber of the second embodiment of Ser. No. 725,509 consists essentially of the hollow fiber of the first embodiment additionally coated on its exterior surface with a coating film of a polymer, said film having a thickness of from 0.1 to 300.mu..
As the cellulose derivative that is used in Ser. No. 725,509, there can be mentioned, for example, cellulose acetate, cellulose acetate butyrate, cellulose propionate and ethyl cellulose. Acetone-soluble cellulose acetate is especially preferred.
Acetone alone or a mixture of acetone with one or more additional solvents such as, for example, dimethyl formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane or 1,3-dioxolan can be used for dissolving such cellulose derivative to form the starting solution for the extrusion step. In order to form fine pores, a swelling agent or an inorganic salt such as potassium hypochlorite may be added to the solvent. The use of an acetone-formamide mixed solvent or acetone-1,3-dioxane mixed solvent is preferred. A cellulose derivative such as mentioned above is dissolved in such solvent so that its concentration in the solution is from 22 to 33% by weight, which provides a viscosity of 300 to 1500 poises (measured at 20.degree. C.) suitable for the formation of fibers. The solution is allowed to stand still to remove gases sufficiently. Then, the solution is fed quantitatively to the annular space between the concentric outer and inner tubes of a spinneret of a double tube structure by a gear pump, and simultaneously, an aqueous liquid compatible with the solvent of the solution, or an inert gas, is fed to the inner tube of the spinneret by a metering pump or the like, and thus, composite spinning of the solution, as the sheath, and the aqueous liquid or gas, as the core, is performed. In order to facilitate the spinning operation, it is necessary to maintain the viscosity of the solution within a certain range. If the viscosity is lower than 300 poises, the solution does not possess a fiber-forming property and because the extrudate is readily broken by a very slight change of the tension, it is impossible to conduct continuous spinning. If the viscosity of the solution is higher than 1500 poises, because the pressure of the spinning solution must be very high before it exits from the spinneret, it is difficult to feed the solution by an ordinary spinning gear pump, and because the extruded fiber cannot be drawn, it is impossible to conduct the spinning operation in a good condition.